1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image fixing apparatus which uses an induction heater and is capable of stably controlling a fixing temperature.
2. Discussion of the Background
A background image forming apparatus such as a copy machine, a printer, a facsimile machine, and a multi-function machine capable of copying, printing, and faxing uses an electromagnetic induction type fixing mechanism to reduce a machine rise time for an energy savings.
One example of the electromagnetic induction type fixing mechanism includes a support roller, an auxiliary fixing roller, a fixing belt, a magnetic flux generator, and a pressure roller. The support roller serves as a heat roller, and the auxiliary fixing roller serves as a fixing roller. The fixing belt has a heat resistant property and is extended between the support roller and the auxiliary fixing roller. The magnetic flux generator faces the support roller via the fixing belt. The pressure roller faces the auxiliary fixing roller via the fixing belt. The magnetic flux generator includes a coil including a plurality of wire turns and a core such as an exciting coil core. The coil is wound around the core and is extended in a direction parallel to a surface of a recording sheet in conveyance and perpendicular to a conveyance direction of the recording sheet which is conveyed between the pressure roller and the auxiliary fixing roller.
The fixing belt is heated at a position facing the magnetic flux generator and applies heat to a toner image carried on a recording sheet which is transported to a nip formed between the auxiliary fixing roller and the pressure roller. More specifically, the coil receives an application of a high-frequency alternating current to generate a magnetic field around the coil. The magnetic field induces an eddy current near a surface of the support roller. This causes a generation of Joule heat due to an electrical resistance of the support roller itself.
The above-described electromagnetic induction type fixing mechanism is capable of increasing a fixing temperature of the fixing belt to a desired level in a relatively short time period and with a relatively small amount of energy.
However, the electromagnetic induction type fixing mechanism cannot make sure to suppress a temperature increase at longitudinal end sides of the fixing member, e.g., the fixing belt or roller, which may be overly heated, especially, when the image forming operation is consecutively performed on a narrower-sized recording sheet.
In general, an image forming apparatus is configured to handle various kinds of recording sheets specially in size for image forming: for example, standard A-series size such as A4, or irregular size as well. A recording sheet in A4 size, for example, is in a rectangular form and has a long side and a short side. Therefore, a surface area of the fixing belt facing the recording sheet can be changed by an orientation of image forming, depending on whether the recording sheet needs to be placed in landscape or portrait relative to the fixing belt.
Such a variation of width of the recording sheet causes the fixing belt to have an uneven temperature in the axis direction thereof. That is, during the fixing process, the recording sheet absorbs a certain amount of heat from the surface area of the fixing belt. This results in an uneven surface temperature of the fixing belt. Specifically, a sheet-contact area of the fixing belt which makes contact with the recording sheet has the temperature decreased and a non-sheet-contact areas around both end sides of the fixing belt which do not make contact with the recording sheet have higher temperatures. This problem occurs typically when the image forming is consecutively performed to a relatively small size recording sheet.
If the surface temperature of the fixing belt is adjusted to attempt to increase the lowered temperature of the sheet-contact area of the fixing belt, the lowered temperature of the sheet-contact area of the fixing belt can be adjusted to an appropriate level; however, at the same time, the temperature of the non-sheet-contact area are may exceedingly be increased. If the image forming operation is performed to a relatively large size recording sheet under this condition, a troublesome phenomenon referred to as a hot off-set may be caused at a surface area of the fixing belt where the fixing temperature is too high. That is, because of the exceedingly high temperature, a portion of toner included in the toner image carried on the recording sheet is melt on the recording sheet and is adhered to the fixing belt, not to the recording sheet. As a result, the toner image on the recording sheet loses a portion thereof. If the temperature is partly risen on the surface of the fixing belt in excess of a predetermined range of the fixing temperature, the fixing belt may cause a thermal breakdown.
In contrast, if the surface temperature of the fixing belt is adjusted to attempt to decrease the exceedingly risen temperature of the non-sheet-contact area of the fixing belt, the exceedingly risen temperature of the non-sheet-contact area of the fixing belt can be adjusted to an appropriate level; however, at the same time, the temperature of the sheet-contact area may exceedingly be decreased. If the image forming operation is performed under this condition, another troublesome phenomenon referred to as a cold off-set may be caused at a surface area of the fixing belt where the fixing temperature is too low. That is, because of the exceedingly low temperature, a portion of toner included in the toner image carried on the recording sheet is not melt on the recording sheet and is adhered to the fixing belt, not to the recording sheet. As a result, the toner image on the recording sheet loses a portion thereof.
One example technique attempts to solve the above-described problems by suppressing an increase of the fixing temperature at the non-sheet-contact area of the fixing roller. This technique provides a magnetic flux shield for shielding a part of the magnetic flux generated by the magnetic flux generator (e.g., an induction coil) disposed inside the fixing roller. More specifically, the magnetic flux generator is configured to change its position in accordance with a sheet-contact area of the fixing roller to change a range of area to shield accordingly so as to shield the magnetic flux applied to the fixing roller at the non-sheet-contact area of the fixing roller. Thereby, a temperature rise at the non-sheet-contact area of the fixing roller is suppressed.